The present invention relates to a dot matrix printer which stores bit map data composed of characters and graphics provided by a user, and which reads out the bit map data with code signals received from a host computer. More particularly, the invention relates to a dot matrix printer employing an improved technique for storing and reading out the bit map data.
To print patterns other than the graphics and characters previously stored as character sets in a ROM of a printer, the user must prepare pattern bit map data corresponding to the characters to be printed, and store the prepared patterns as supplemental character data in a memory of the printer.
Usually, a part of a buffer memory, for example, in the printer, is used for storing the supplemental character data. In this case, it is necessary to provide in advance a memory having a capacity sufficient for storing the data of the largest size expected.
The amount of supplemental character data differs among users. Some users require only a small amount of supplemental character data, while others need a large amount of such data. In the former case, a large part of the memory allocated for storing the supplemental character data is not used and is wasted.
A technique for reducing the memory capacity of the memory used for storing the data by compressing the bit map data of the supplemental character data has been used for factory-installed character patterns. However, it is difficult for general users to employ such a method for storing supplemental character data since special knowledge is required for carrying out such a method.